Maximum thrust and efficiency of a gas turbine engine is achieved when the exhaust passes through a discharge nozzle which controls expansion and maximizes the discharge velocity. When an aircraft operates at both subsonic and supersonic speeds the exhaust nozzle pressure ratio varies over a substantial range.
Under subsonic flight conditions the pressure ratio is relatively small and a nozzle having substantially a convergent shape is desirable. At supersonic flight conditions when the nozzle pressure ratio is high the appropriate geometry is achieved by a nozzle having a convergent portion followed by a divergent portion. This is referred to as a convergent divergent nozzle.
Many designs have been made which provide variable geometry to effect proper operation at both subsonic and supersonic speeds. For the subsonic condition where only the convergent nozzle is desired, there is only a nominal divergence, this being selected to assure that the throat area remains upstream of the divergent flaps. At supersonic speeds means are supplied to effect the appropriate divergent flow path downstream of the throat. Many of these designs require additional structure and weight to achieve the actuation of the divergent flaps. In an attempt to save the weight and decrease the complexity of the drive mechanism limited range floating divergent flaps have been used. Such a flap is illustrated in U.S. Pat. No. 3,792,815.
The divergent flaps are supported by struts including a lost motion connection. The lost motion connection allows movement between a low mode less divergent position and a high mode more divergent position. An internal/external gas pressure force unbalance establishes the actual location of the flap.
Such a structure works effectively at low speeds and at supersonic speeds. In between there are situations where the flap has not achieved the optimum position for the particular flight condition. If the flight condition is only temporary, this is of little concern. However, should this less than appropriate position occur during a design cruise condition a substantial penalty is paid in loss of efficiency. It would be desirable to avoid operation in this poor efficiency condition.
The pressure existing within the divergent nozzle firmly secures the floating flap in the low mode at low speeds and in the high mode at high supersonic speeds. At intermediate conditions the flap floats between the two extremes. When the pressure increases enough to move the flap away from the low mode position the increased divergence changes the static pressure against the flap so that the flap substantially settles at an equilibrium position between the two extremes. The resulting divergence set by the force balance during this lightly loaded condition can be less than ideal and can result in over expansion, flow separation and a net drag increase (negative thrust). Minor but constant pulsations in the gas flow during this near balance condition can cause a constant flutter of the divergent nozzle elements in this condition, concommitant wear of the linkage, and thrust pulsations. Asymmetric pressure distribution in the nozzle can cause ovalization of the nozzle resulting in varying divergence of the various flaps. This condition varies the performance and durability of floating divergent nozzle designs. When these conditions occur during aircraft cruise, the actual time the nozzle is subject to these conditions is significant.
During various aircraft maneuvers the G-forces and flow field pressures operating on the aircraft cause variation in the position of the divergence elements, which variation should be preferably minimized.